1. Field of the Invention
The present invention is in the field of fabrication of semiconductor devices. More specifically, the invention is in the field of fabrication of resistors for integrated circuits.
2. Related Art
The integrated circuit (“IC”) chips in modern electronic devices include circuits, such as mixed signal circuits, RF circuits, and power amplifier circuits, that require accurate and stable voltages to operate properly. For example, a change in reference voltage can cause the bias voltage in a power amplifier circuit to change, which may result in undesirable distortion in an output signal of the power amplifier circuit. To obtain stable voltages, the circuits in an IC chip, in turn, require components, such as resistors, that remain stable under diverse operating conditions. Furthermore, the circuits in an IC chip require voltages that remain stable while the IC chip is operating in varying ambient or junction temperatures. Thus, semiconductor manufacturers are challenged to fabricate resistors that provide stable resistance values under varying temperatures.
In a conventional IC chip, polysilicon (“poly”) resistors are commonly used, and may be fabricated using metal oxide semiconductor (“MOS”) technology. For example, a polysilicon resistor may be fabricated by depositing a polysilicon film on a field oxide region in the IC chip. The polysilicon film may be deposited, for example, using a low-pressure chemical vapor deposition (“LPCVD”) process. The deposited polysilicon film may then be patterned and etched to form a resistor. The resulting poly resistor has a negative temperature coefficient, which means that the resistance of the poly resistor will decrease in value as temperature increases. A poly resistor, for example, may have a temperature coefficient of approximately −300.0 parts per million (“ppm”)/° C.
The temperature coefficient of the poly resistor, and thus the sensitivity of the resistance of the poly resistor to temperature variations, may be reduced by increasing the size of the poly resistor. For example, a poly resistor having a width of 2.0 microns and a length of 10.0 microns may have a temperature coefficient equal to approximately −350.0 ppm/° C. By increasing the width and length, respectively, of the poly resistor to 4.0 microns and 20.0 microns, for example, the temperature coefficient of the poly resistor may be reduced to approximately −200.0 ppm/° C. Although increasing the size of the poly resistor reduces the sensitivity of the poly resistor to temperature variations, it also results in an undesirable consumption of space in the IC chip.
Metal resistors are also utilized by semiconductor manufacturers in IC chips to provide stable resistances and thus, accurate operating voltages, such as reference and bias voltages. Conventional metal resistors typically have a positive temperature coefficient, which causes an increase in resistance as temperature increases. Similar to poly resistors, metal resistors may also be increased in size to reduce their temperature sensitivity. However, as with poly resistors, increasing the size of the metal resistor also results in an undesirable consumption of space in the IC chip.
Thus, there is a need in the art for a resistor that achieves a stable resistance under varying temperature conditions without consuming additional space in an IC chip.